In the recent years, there have been prevailing TV systems for endoscopes which are configured so as to permit observing images of coelomata, etc. on TV monitor and the similar apparatus. Since such a TV system allows an image to be observed simultaneously by a large number of persons at the same time and specific information contained in an image of object to be conspicuous or to be erased by utilizing image processing techniques, these TV systems are utilized for diagnoses of living bodies, surgical operations, various types of inspections in body cavities, and will find wider fields of application in the future.
There is known, as an example of surgical operations using endoscopes, laparoscopic chole-cyst-ectomia which is carried out for removing chole-lithus. Recently, attention has been paid to a surgical operation which is referred to as "Common Bile Scopy". This operation is adopted for removing chole-lithus after switching a rigid endoscope used for observation to a fiber scope which is capable of allowing a scalpel, scissors and other means for surgical operations to protrude out through a forceps channel when chole-lithus filling a bile duct is found during the laparoscopic chole-cyst-ectomia.
An example of a TV system for endoscopes which is used for the surgical operation will be described with reference to FIG. 1 through FIG. 4. In FIG. 1, the reference numeral 1 represents a fiber scope which uses an image guide fiber bundle as an image transmission system, the reference numeral 3 designates a rigid endoscope which uses a relay lens system as an image transmission system. The reference numeral 5 represents objective lenses which are disposed in distal ends of the fiber scope 1 and the rigid endoscope 3 respectively, the reference numeral 6 designates eyepiece lenses disposed in eyepiece units 7 of the fiber scope and the rigid endoscope respectively, and the reference numeral 8 denotes an adaptor which comprises an imaging lens system 9 and is adopted for connecting the eyepiece units 7 to a TV camera 10 which does not comprise an imaging optical system. Disposed in the TV camera 10 are an optical low-pass-filter 11 and a solid-state image pickup device (CCD) 12.
When the fiber scope 1 is selected as an endoscope for observation, an object M is illuminated with an illumination light bundle which is emitted from a light source (not shown) and transmitted through the light guide, an image of the object M is formed by the objective lens 5, and this image is transmitted through the image guide fiber bundle 2. The image is reimaged on the solid-state image pickup device 12 through the eyepiece lens system 6, the imaging lens system 9 and the low pass filter 11, and reproduced on a TV monitor through a camera control unit 13. When the rigid endoscope is selected as an endoscope, the image of the object M is transmitted through the relay lens system 4, reimaged on the solid-state image pickup device 12 through the eyepiece lens system 6, the imaging lens system 9 and the low pass filter 11 as in the case where the fiber scope 1 is selected, and reproduced on the TV monitor 14.
FIG. 2 illustrates a system configuration wherein the adaptor 8 is replaced with a camera head 15 which comprises the imaging lens system 9, the low pass filter 11 and the solid-state image pickup device 12. Further, FIG. 3 shows a system configuration which allows only the fiber scope 1 to be connected to the camera head, whereas FIG. 4 illustrates another system configuration wherein the eyepiece lens system 6 is not disposed in the fiber scope 1. In the system configuration illustrated in FIG. 4 wherein a camera head 13a consisting of the imaging lens system 9, low pass filter 11 and the solid-state image pickup devices 12, and a signal processing circuit 13b are disposed in the camera control unit 13, the fiber scope 1 is to be connected directly to the camera control unit 12, differently from the case of the system configurations which have already been described above. The fiber scope may be a fiber scope which uses a flexible image guide fiber bundle and is flexible as a whole or a fiber scope which uses a rigid image guide fiber bundle and is not flexible as a whole.
In each of the TV systems for endoscopes described above, an image formed on the solid-state image pickup devices i.e., an image transmitted to the solid-state image pickup device through the fiber scope or the rigid endoscope is used as an image to be projected to the TV monitor. An image formed by the fiber scope particularly has a characteristic described below. FIG. 5 shows an end surface of emergence of an image guide fiber bundle in which optical fibers are arranged densely in six directions. In FIG. 5, the reference numeral 2a represents a core of an image guide fiber bundle 2, and the reference numeral 2b designates a cladding.
Further, the arrow A indicates a direction in which the optical fibers are arranged and the arrow B indicates a horizontal scanning direction of a solid-state image pickup device 12 on which elements are disposed in a lattice pattern. When an image of the end surface of emergence of an image guide fiber bundle is picked up by the solid-state image pickup device 12, signals of the image formed by the endoscope contain an intense frequency spectrum fF=1/Pf.times..beta..times.sin 60.degree.) which is produced due to arranged patterns on the image guide fiber bundle 2 and the solid-state image pickup device 12. In the formula expressing the intense frequency spectrum, the reference symbol Pf represents a pitch of images of the optical fibers and the reference symbol .beta. designates a magnification of the imaging optical system. This frequency spectrum fF is an especially intense frequency spectrum of the first order and, when it is illustrated on a two-dimensional spatial frequency plane taking the arrangement direction of the optical fibers as a horizontal axis, a large number of discrete frequency spectra of the second and higher orders appear as shown in FIG. 6.
When an image to be projected to the TV monitor is an image of an end surface of emergence of an image guide fiber bundle in which optical fibers are arranged at random as illustrated in FIG. 7, a frequency spectrum produced due to the arrangement pattern of the optical fibers is expressed as fF=1/(Pf'.times..beta.). In this formula, the reference symbol Pf' represents an average pitch of the optical fibers arranged at random. In this case, the frequency spectra are distributed nearly uniformly in all directions, unlike the case where the optical fibers are arranged densely in six directions.
In an optical instrument which discretely samples spatial frequencies for picking up an image by utilizing the solid-state image pickup device, high-frequency components exceeding the Nyquist frequency limit, if contained in the images of the object, produce spurious signals called aliasing, moire, etc. due to beat between the high-frequency components and a sampling frequency.
Conventionally, these spurious signals are eliminated by disposing an optical low pass filter between the imaging optical system and the solid-state image pickup device as described above.
When an image of an object obtained with a fiber scope or a similar optical instrument is to be picked up by using a TV camera, however, it is necessary to sufficiently lower the response of the imaging optical system by adopting an optical low pass filter which is composed of a large number of birefringent plates as described in detail in Japanese Patent Preliminary Publication No. sho 63-291026.
In the TV system for endoscopes which can use the fiber scope 1 and the rigid endoscope 3 selectively as illustrated in FIG. 1 and FIG. 2, the optical low pass filter 11 is commonly used for the fiber scope 1 and the rigid endoscope 3. When an optical low pass filter having such a characteristic as to eliminate the moire which is produced by selecting the fiber scope 1 is adopted for the TV system for endoscopes, the resolution of an image projected to the TV monitor is degraded. When an optical low pass filter having such a characteristic as to enhance the resolution of images formed by the rigid endoscope, image quality is degraded by the moire which is produced by selecting the fiber scope.
For this reason, degradation in quality of images formed by the fiber scope 1 or the rigid endoscope 3 is ignored in the TV system for endoscopes described above or the imaging lens system 9 is slightly defocused to prevent the production of the moire only when the fiber scope is selected as described in Japanese Patent Preliminary Publication No. Hei 2-89225.
A method which utilizes the digital technique successfully for obtaining, on a TV monitor, images which are formed with endoscopes, free from moire and noise, and high in resolution and contrast will be described below.
FIG. 8 is a block diagram of main components of the camera control unit 13 which uses conventional analog circuits. In the camera control unit 13 shown in this drawing, an electrical signal output from a solid-state image pickup device 12 disposed in a TV camera 10 is divided into an electrical signal representing luminance information and another electrical signal representing color information while passing through an electrical low pass filter 16 and an electrical bandpass filter 17, respectively, which are disposed in parallel with each other in the camera control unit 13. The luminance information is divided by an aperture signal generating circuit 18 into an electrical signal for generating an aperture signal for emphasizing edges and into another electrical signal which is to be used as a luminance signal in a TV signal. Further, the electrical signal representing the color information is modulated by a modulator circuit 19 into a color difference (chroma) signal in the TV signal.
In a camera control unit which uses the digital circuits illustrated in the block diagram shown in FIG. 9, the electrical signal output from the solid-state image pickup device is divided into an electrical signal providing luminance information and another electrical signal providing color information as in the case of the analog circuits described above. These electrical signals are converted into digital signals by digital circuits 20 so that noise is not produced by the subsequent circuits and so that the S/N ratio is enhanced. A function of the aperture signal which is divided from the luminance information by an aperture signal generating circuit 18 will be described conceptionally with reference to FIG. 10A, FIG. 10B and FIG. 10C. Assuming that the rectangular wave shown in FIG. 10A is the electrical signal providing the luminance information, an aperture signal shown in FIG. 10B is obtained by processing this rectangular signal. By overlapping this aperture signal with the luminance signal, a signal emphasizing edges as shown in FIG. 10C (the aperture signal) can be obtained. Since the signal shown in FIG. 10C apparently has negative spot image intensity distribution, apparent response is changed from that shown in FIG. 11A to that shown in FIG. 11B exceeding 100%, whereby response, or contrast of the image of the object, can be enhanced.
The camera control unit illustrated in FIG. 9 comprises a color signal suppressing circuit which utilizes the digital signal processing technique described above. After the aperture signal shown FIG. 10B is generated by the aperture signal generating circuit 18, an absolute value of an amplitude of the aperture signal is calculated by a circuit 21, and an output intensity of the electrical signal providing the color information is adjusted in accordance with the absolute value by a chromagain controller 22 (these circuits and controller compose an aperture control circuit), and the color difference signal output from the modulator circuit is suppressed by a chromasuppress circuit 23. These circuits comprise the chromasuppress circuit 23. When high-frequency components producing moire are contained in the image of the object, the aperture signal generated from the high-frequency components has a wide amplitude and the color signal suppressing circuit operates to eliminate moire produced due to the high-frequency components.
Accordingly, a camera control unit 13 which comprises the color signal suppressing circuit utilizing the digital technique as described above has improved resolution since it is unnecessary to eliminate the moire by using the optical low pass filter or slightly blurring the image as in the conventional TV system for endoscopes. Consequently, the camera control unit 13 makes it possible not only to provide an image which is not inferior in contrast to the image obtained by the conventional analog TV system for endoscopes but also enhances the S/N ratio and reduces noise owing to the digital circuits. Though this color signal suppressing circuit can be composed of conventional analog circuits, the color suppressing circuit operates more stably when it uses digital circuits for processing the digital signals.
Since frequencies which can be resolved by a fiber scope are determined dependently on an arrangement pitch of the fibers in the image guide fiber bundle used as the image transmission system, it is meaningless to improve the resolution of the solid-state image pickup device so as to be higher than the resolution of the image guide fiber bundle. With the rigid endoscope which uses the relay lens system as the image transmission system, however, enhancement of the resolution of the solid-state image pickup device directly results in improvement of quality of the images formed by the endoscopes. In the TV system for endoscopes described above, it is therefore possible to obtain images of very high quality when the endoscope is selected and images of quality equal to that of the images obtained by the conventional TV system for endoscopes when the fiber scope is used, thereby making it possible to compose a TV system for endoscopes featuring higher accuracy which permits performing the above-described surgical operation more accurately and more speedily.
When the image guide fiber bundle is used as the image transmission system as in the case of the fiber scope, however, an object to be photographed by a TV camera is an end surface of emergence of the image guide fiber bundle which consists of cores 2a and claddings 2b as illustrated in FIG. 12A and FIG. 12B, whereby a difference in brightness is clearly produced between the images of the cores 2a and the clads 2b as shown in FIG. 12C and FIG. 12D in practical use of the TV system for endoscopes, and the image of the end surface contains a large number of high-frequency components.
Accordingly, the aperture signal has a very high intensity and the color signal suppressing circuit adjusts the electrical signal providing the color information to lower an output intensity thereof. For this reason, the images obtained by the fiber scope become images on which colors are faded on the TV monitor.
When the images are picked up by a single-plate type solid-state image pickup device having a color encoding filter which is disposed on a light receiving surface, and has color filters of G (green), C (cyanogen), Mg (magenta) and Y (yellow) arranged in a lattice pattern thereon (FIG. 13), sampling points are formed at (1/2Px, 0), (1/2Py, 0), (1/2Px, 1/4Py), (-1/2Px, -1/4Py), (1/2Px, -1/4Py) and (-1/2Px, 1/4Py), wherein the reference symbols Px and Py represent the pitch of picture elements of the filter, with respect to the horizontal scanning direction and to the vertical scanning direction, respectively, on a coordinates system on the two-dimensional spatial frequency plane illustrated in FIG. 14. When an image of the end surface of emergence of the image guide fiber bundle in which optical fibers are arranged densely in six directions as described above is picked up by using these filters, a frequency spectrum fF of the first order, contained in the image, exists in the vicinity of the sampling point as shown in FIG. 5, intense moire is produced in this vicinity. In FIG. 15, the frequency spectrum is illustrated as that having certain width on an assumption that the imaging lens system is a zoom lens system.
In order to eliminate the moire, the intensity of the frequency spectrum fF which exists in the vicinities of the sampling points is conventionally attenuated by using a large number of birefringent plates as it is disclosed by Japanese Patent Preliminary Publication No. Hei 1-284225.
In recent years where picture elements are disposed at higher densities in solid-state image pickup devices, a Serious problem is posed by moire which is produced by the color sub-carrier frequency. Each of the NTSC,
and SECAM systems adopt a composite signal output system which outputs the luminance signal and the color difference signal at the same time. In an NTSC system, for example, signals to be output for a single scanning line on the TV monitor have a waveform illustrated in FIG. 16A. The TV monitor judges a color phase from a phase difference between a color burst signal (signal frequency 3.58 MHz) and the color difference signal (color sub-carrier frequency 3.58 MHz), and chroma from an amplitude of the color difference signal. When the luminance signal contains components which have frequencies in the vicinities of 3.58 MHz, or nearly equal to the color sub-carrier frequency of the color difference signal (see FIG. 16B), the TV system may regard these components as the color difference signals and allow colors different from those on an original image to be displayed on the TV monitor. FIG. 17 illustrates the color sub-carrier frequency represented on a two-dimensional frequency plane.
In the conventional TV system for endoscopes wherein picture elements are arranged coarsely in the solid-state image pickup device and the sampling point has a frequency close to the color sub-carrier frequency, the frequency spectrum of the optical fibers existing in the vicinity of the color sub-carrier frequency can be attenuated by using an optical low pass filter which lowers the intensity of the frequency spectrum of the optical fibers existing in the vicinity of the sampling point.
When an image formed by a fiber scope is picked up by using a solid-state image pickup device in which picture elements are arranged densely, however, moire is produced on the picked up image due to interference between the intense frequency spectrum fF contained in the image of the end surface of the image guide fiber bundle and the color sub-carrier frequency. In this case, the conventional optical low pass filter described above poses a problem in that it cannot sufficiently attenuate the intensity of the frequency spectrum fF existing in the vicinity of the color sub-carrier frequency, thereby being incapable of preventing moire from being produced.
Further, the TV system for endoscopes which permits connecting the fiber scope 1 and the rigid endoscope 3 selectively as illustrated in FIG. 1 and 2 poses a problem in that the resolution is degraded when the rigid endoscope is used since this TV system usually comprises an optical low pass filter for the fiber scope and images formed by the rigid endoscope must be picked up through this optical low pass filter.
More recently, on the other hand, there has been proposed a camera control unit which allows, when an image of an object containing frequency components exceeding the color sub-carrier frequency causing moire is picked up, the color signal suppressing circuit functions in accordance with a value of an amplitude of the aperture signal generated from the high-frequency components so that a color output corresponding to the high-frequency components is lowered making color moire less noticeable. When an image of the end surface of emergence of the image guide fiber bundle shown in FIG. 6 and FIG. 7 is picked up by using this camera control unit, however, there is a problem in that the aperture signal has a high intensity due to a very large number of high-frequency components contained in the image, whereby the color signal suppressing circuit functions to lower the output intensity of the color information signal as if a non-chromatic image were picked up and a sharp image cannot be obtained.